A team from the University of California Riverside and the California Air Resources Board (ARB) has evaluated California and Federal diesel fuels with heavy-duty engine and chassis dynamometer tests and concluded that aftertreatment systems for PM and NOx will, over time, largely eliminate any potential benefits that might be obtained through the use of CARB diesel, although NOx benefits will persist through to 2020.

The ARB has regulated the properties of diesel fuel sold in California since 1988 to lower emissions of particulate matter (PM) and oxides of nitrogen (NOx). Although many studies have shown that reduced levels of aromatics and higher cetane numbers can improve emissions, the actual impact of CARB fuels on in-use diesel emissions has not yet been extensively studied, especially as diesel engine and aftertreatment technology has evolved over the years.

Other findings of the study, published in the International Journal of Engine Research, include:

Engine dynamometer results showed that NOx emissions for the Federal fuels ranged from 4.7% to 9.5% higher than the CARB diesel. These NOx reductions are similar to the estimates being used in the latest regulations.

The chassis dynamometer test results did not show as consistent trends for NOx as those seen for the engine dynamometer testing. For the chassis dynamometer testing, four out of ten vehicles showed consistent reductions in NOx, with emissions for the Federal fuels ranging from 3.3% to 9.9% higher than the CARB diesel, while the other six vehicles did not show consistent fuel impacts.

On an absolute level, the NOx benefit for CARB diesel shows a decline with continuing advances in engine technology.

CARB diesel did not show strong benefits for PM.

Resources

Maryam Hajbabaei et al. (2012) Assessment of the emissions from the use of California Air Resources Board qualified diesel fuels in comparison with Federal diesel fuels. International Journal of Engine Research ddoi: 10.1177/1468087412446883